Colors are imparted to various media by dissolving and/or dispersing organic dyes in them, or by suspending insoluble inorganic pigments in the media during their manufacture. Colored media may serve as the material out of which an end product is fabricated (e.g., in the case of plastic articles), or may be applied to a substrate in the form of an ink, paint or toner. Electrophoretic toners, for example, consist of pigments dispersed in an oil-based, low-viscosity carrier. The pigments are given electrical charges so they can be attracted by electrical charges of opposite polarity on a substrate. During an imaging procedure, the pigment particles migrate, driven by electrostatic attraction, towards the substrate surface in response to an imagewise electrical charge pattern applied thereto.
Pigments may also be retained within a permanent liquid carrier to form displays. An electrophoretic display utilizes charged particles of one color suspended in a dielectric liquid medium of a different color. The suspension is housed in a cell located between (or partly defined by) a pair of oppositely disposed electrodes, one of which is optically transmissive. When the electrodes are operated to apply a DC or pulsed field across the medium, the particles migrate toward the electrode of opposite sign. The result is a visually observable color change. In particular, when a sufficient number of the particles reach the optically transmissive electrode, their color dominates the display; if the particles are drawn to the other electrode, however, they are obscured by the color of the liquid medium, which dominates instead.
As a rule, pigments consist of poorly dispersed, agglomerated discrete particles whose approximate coloring effects are derived from the widely varying shape and size distributions throughout the carrier material. Different colors and hues are obtained by mixing quantities of different color pigments. It has been found, however, that the purity and predictability of color relates quite specifically to the particular size, shape, and morphology of the pigment particles. For demanding colorant applications, it is desirable to utilize pigment particles exhibiting a narrow size distribution (i.e., which are monodispersed), identical shape (frequently, but not always spherical), identical bulk properties, and identical surface electrical properties (directly or through additives). Heretofore, such uniform synthetic pigment particles have been difficult to manufacture in a controllable, inexpensive fashion.
At the same time, while pigment particles have been utilized in electrophoretic displays, many common forms of electronic display require larger and/or specialized elements that are expensive to fabricate. It would be beneficial to utilize the versatility of pigments in connection with the many types of display and information-bearing devices that currently require specialized components.